1. Field of the Invention
The present invention relates to a fluorescence sensor that is a minute-fluorescence photometer for measuring the concentration of an analyte in a living body, a needle-type fluorescence sensor including the fluorescence sensor, and a method for measuring the analyte by using the fluorescence sensor, and particularly to a fluorescence sensor fabricated by using semiconductor manufacturing technologies and micro-machine manufacturing technologies, a needle-type fluorescence sensor including the fluorescence sensor, and a method for measuring the analyte by using the fluorescence sensor.
2. Description of the Related Art
A variety of analyzing apparatuses have been developed to check the presence of an analyte, that is, a substance to be measured in a liquid, or measure the concentration thereof. For example, there has been a known fluorescence photometer that quantifies the concentration of an analyte by injecting a solution to be measured containing a fluorescent dye and the analyte, the fluorescent dye changing its nature in response to the presence of the analyte to emit fluorescence, into a fixed-capacity, transparent container, irradiating the container with excitation light, and measuring the intensity of the fluorescence from the fluorescent dye.
On the other hand, a small-sized fluorescence photometer specializing in specific analyte detection includes a light source, a photodetector, and an indicator layer containing a fluorescent dye that interacts with a specific analyte in a solution to be measured. When the indicator layer, which the analyte in the solution to be measured can enter, is irradiated with excitation light from the light source, the fluorescent dye in the indicator layer emits fluorescence, the amount of which depends on the concentration of the specific analyte in the solution to be measured, and the photodetector receives the fluorescence. The photodetector is a photoelectric conversion device and outputs an electric signal according to the amount of received light. The electric signal is used to determine the concentration of the analyte in the solution to be measured.
In recent years, to measure a specific analyte in a tiny amount of specimen, a minute-fluorescence photometer fabricated by using semiconductor manufacturing technologies and micro-machine manufacturing technologies has been proposed.
For example, U.S. Pat. No. 5,039,490 discloses a fluorescence sensor 110 shown in FIGS. 1 and 2. It should be noted that an analyte 2 is diagrammatically illustrated in the following figures.
As shown in FIGS. 1 and 2, the fluorescence sensor 110 includes a transparent support substrate 101 through which excitation light E from an external light source can pass, a photoelectric conversion device unit 103, which is a photodetector for converting fluorescence F into an electric signal, an optical plate-shaped unit 105 having a light collecting function section 105A for collecting the excitation light E, an indicator layer 106 that interacts with the analyte 2 to emit fluorescence F when the excitation light E is incident, and a cover layer 109.
The photoelectric conversion device unit 103 has a photoelectric conversion device formed in a substrate 103A made, for example, of silicon. The substrate 103A does not transmit the excitation light E. In consideration of this fact, the fluorescence sensor 110 has a void region (transparent zone) 120, through which the excitation light E can pass, around the photoelectric conversion device unit 103.
That is, only the excitation light E that passes through the void region 120 and impinges on the optical plate-shaped unit 105 is collected by the effect of the optical plate-shaped unit 105 in the indicator layer 106 in the vicinity of an upper portion of the photoelectric conversion device unit 103. The collected excitation light E2 interacts with the analyte 2 that has entered the indicator layer 106, and fluorescence F is produced. Part of the produced fluorescence F is incident on the photoelectric conversion device unit 103, which produces a current or voltage signal or any other suitable type of signal proportional to the intensity of the fluorescence, that is, the concentration of the analyte 2. It should be noted that the excitation light E is not incident on the photoelectric conversion device unit 103 due to the effect of a filter (not shown) disposed on the photoelectric conversion device unit 103.
As described above, the fluorescence sensor 110 is configured in such a way that a photodiode, which is the photoelectric conversion device unit 103, is formed on the transparent support substrate 101 and in the substrate 103A having the void region 120, which is a passage of the excitation light E, and the optical plate-shaped unit 105 and the indicator layer 106 are disposed above the substrate 103A.
In the known fluorescence sensor 110 described above, however, the void region 120, which is a passage of the excitation light E, and the photoelectric conversion device unit 103 are disposed in the same plane. As a result, increasing the area of the void region 120, which is a passage of the excitation light E, to introduce more excitation light E into the indicator layer 106 results in decrease in the area of the photoelectric conversion device unit 103 and hence does not increase the sensitivity of the fluorescence sensor. Conversely, increasing the area of the photoelectric conversion device unit 103 to increase the detection sensitivity thereof results in decrease in the area of the void region 120, which is a passage of the excitation light E, and hence decrease in the amount of excitation light E to be introduced into the indicator layer 106, which does not increase the sensitivity of the fluorescence sensor either. That is, in the fluorescence sensor having the laminate structure described above, it is difficult to achieve high detection sensitivity.
On the other hand, a needle-type sensor is a sensor for determining the concentration of an analyte, that is, a substance to be measured, in the blood or a body fluid in a subject by puncturing the subject with a needle distal end portion so that a sensor unit, which is a fluorescence sensor, is inserted into the body of the subject. A short-term indwelling, needle-type sensor can continuously determine the concentration of a substance to be measured in the body of a subject for a predetermined period, for example, one week.
The needle-type fluorescence sensor shown in FIGS. 3 and 4 is a biosensor 210 disclosed in International Publication No. 06/090596. It should be noted that an analyte 2 is diagrammatically illustrated in the following figures. The biosensor 210 includes a needle-shaped hollow container 212, a carrier tube 214 inserted into the hollow container 212, and an optical fiber 218, an end portion of which is inserted into the carrier tube 214. The hollow container 212 has one sharp end and one open end. A plurality of through holes 220 are provided through a side portion of the hollow container 212. The carrier tube 214 is formed of a rounded thin film. A sensor unit 216 is formed only of an indicator section, which is an end portion of the optical fiber 218 covered with a ruthenium organic complex thin film 224, which is a fluorescent dye.
The analyte 2 enters the sensor unit 216 through the through holes 220. The biosensor 210 uses the optical fiber 218 to irradiate the sensor unit 216 with excitation light from a light source (not shown) external to the biosensor 210, uses the optical fiber 218 to receive fluorescence, the amount of which depends on the concentration of the analyte, produced in the sensor unit 216. A photodetector (not shown) external to the biosensor 210 analyzes the fluorescence.
On the other hand, U.S. Pat. Nos. 7,388,110 and 7,524,985 disclose saccharide measuring sensors that are implanted in the body of a subject. Each of the sensors uses a fluorescence sensor substance and an indicator unit using the fluorescence sensor substance. The fluorescence sensor substance is obtained by copolymerizing a fluorescent monomer compound and a polymerizable monomer, the fluorescent monomer compound having a hydrophobic moiety which is bonded to a saccharide to emit fluorescence and to which a hydrophilic group is introduced and the polymerizable monomer having a (meth)acrylamide residue.